"METHOD FOR THE MANUFACTURE OF SINTERABLE MATAL MOULDED PARTS FROM A METAL POWDER"

Abstract

A process for producing sinterable, metallic shaped parts from a metal powder mixed with an auxiliary compacting agent, which contains at least in part components from the family of polyethylene glycols having a molecular weight of between 100 and 6,500 g/mol and which is filled into a compacting mold and, after being compacted under pressure, is ejected as a compacted shaped part from the mold.

Full Text

A process for producing sinterable, metallic shaped parts from a metal powder.
In the manufacture of metal moulded parts using powder-metallurgical processes, one difficulty is to manufacture the moulded parts with as high a density as possible, since the metal powders are first poured into a compacting die and then have to be compacted using high pressure by single-axis or multiple-axis pressing by means of hydraulic or mechanical pressing means. A moulded body obtained thereby, generally referred to as a green compact, is then sintered in a thermal process, usually under a protective atmosphere, so that a solid metal moulded part which is of accurate shape is produced.
: The density of the finished sintered moulded part in this case depends essentially on the green density obtained, the metal powder particles, unlike in the compaction of ceramic powders, undergoing plastic deformation owing to their different crystalline structure and the number of mobile lattice construction defects which this involves. Owing to the particle geometry - likewise unlike ceramic powders - in metal powders the capacity of the individual powder particles to slide against one another is reduced, so that even the loose bed in the pressing mould has a pore volume which during pressing can be overcome virtually completely only by applying very high compaction pressures. High compaction pressures, however, result in a high degree of wear of the pressing tool during the compaction operation, and also result in increas.ed sliding friction upon ejection of the finished green compact in the compacting die, so that here higher
ejection forces with correspondingly increased wear likewise also have to be applied. High ejection forces, however, involve the risk of undesirable localised recompaction and of cracking of the green compact.
In order to avoid these disadvantages, in EP-A-
a method is proposed in which the metal powder to be compacted has added to it a lubricant liquefied by a liquid solvent. Metal stearates, in particular lithium or zinc stearates, and also paraffins, waxes and natural or synthetic fat derivatives, which are first liquefied, for example, with organic paraffin solvents as liquid solvent, are proposed as lubricants for this purpose. The disadvantage of this method is firstly that the dry metal powders first have to be mixed with a dual-component lubricant system, namely the stearates and the solvent, this preliminary mixing having to be largely homogenous. One further disadvantage is that this powder mixture first has to be preheated to a relatively high temperature, into the region of the softening point of the lubricant used before the pressing mould is filled. This means that there is also the risk of baking on in the means feeding the pressing mould. Once the pressing operation has ended and the green compact has been ejected, the lubricant has to be evaporated off in a separate operation before the green compact can then be heated to the actual sintering temperature. In this case, it cannot be avoided that the remaining contents of the lubricant remain in the sintered body, which, depending on the intended use and the type of pure or alloyed metal powder used, may likewise have disadvantages.
A metallurgical powder composition based on iron is known from EP 0 559 987, which composition contains an
organic binder for the iron-based powder contents and the alloy, powder contents. In order to improve the compaction behaviour, the organic binder has a content of polyalkylene oxide-, which should have a molecular weight of ^at-least 7000 g/mol,; but substantially higher molecular weights are preferred.
It'is an object of the invention to improve the methods described above.
The present invention relates to a process for producing sinterable, metallic shaped parts from a metal powder mixed with fctn auidiiaay sompaeting agcn|:, which contains at least in part components from the family of polyethylene glycols. having a molecular weight of between 100 and 6,500 g/mol and which is filled into a compacting mold and, after being compacted under pressure, is ejected as a compacted shaped part from the mold.
The use of pressing aids which have at least contents from the family of the polyalkylene oxides, in particular of the polyalkylene glycols, preferably of the polyethylene oxides, in particular in the form of polyethylene glycols, has surprisingly shown that very much lower compaction pressures can be applied in order to achieve high densities and high green strengths compared with other pressing aids, and that the forces necessary for ejecting the compacted moulded part from the pressing mould are also considerably reduced, so that the disadvantages of the methods previously known listed above are avoided. No particular binder is required in the powder mixture, since owing to the "lubrication" of the powder particles which move relative to one another during the pressing operation already a high strength of the green compact can also be achieved owing to the very much higher "packing density" of the powder particles, and hence an increase in direct contacts between the metal particles in the powder, in addition to a high density. A high green
strength is always desirable when the green compact is to be subjected to additional machining before sintering. "Metal powder" in the spirit of the invention designates the power mixture provided for the manufacture of the moulded part, with all the alloying additions and other additions, with the exception of pressing aids.
One particular advantage of the pressing aids selected from the family of the polyethylene oxides, in particular when they are used in the form of polyethylene glycols, is that the pressing parameters can be influenced by means of a corresponding selection of the molecular weight, both in terms of the flow behaviour during mixing and when filling the mould, and in terms of the softening point and hence of the temperature control and the material flow during the pressing operation. In this case, it is particularly advantageous if the softening point of the pressing aid proposed according to the invention lies between 40gC and BOgC, so that for example in the case of series production the mould temperature which occurs upon continuous pressing as a rule is sufficient to effect perfectly satisfactory "flowing" of the powder mixture upon pouring into the pressing mould and during pressing. Accordingly, the metal powder to which the pressing aid has been added can be poured into the pressing mould at room temperature. In particular in the case of series production, it may be expedient to heat the pressing tool correspondingly in order to offset any interruptions in the series operation. Controlled heating of the pressing tools to about 55gC is expedient, so that both heating due to frictional heat and cooling due to interruption of operation can be taken into account and thus constant pressing conditions can be set. This considerably simplifies
the handling of the metal powder, in particular the filling process, since it is possible to operate using "cold" powder, i.e. with powder at room temperature. Baking-on, lumping or the like cannot occur, since the heating of the metal powder to which the pressing aid has been added does not take place until in the pressing mould. In the case of parts of extremely large volume, additional preheating of powder may be expedient.
The further advantage of the low softening temperature is that immediately after pouring-in first the pressing aid contents in the quantities of metal powder in contact with the heated mould walls reach their softening temperature, so that during the subsequent pressing operation the relative movements between the powder filling and pressing tool occurring at the mould walls already take place in "lubricated" manner and thus the friction is reduced in these regions. Upon the subsequent complete introduction of pressure, the entire powder filling is heated to beyond the softening point owing to the compaction pressure, so that even the internal relative movements in the metal powder filling which are relatively large due to the particle geometry of the metal powder are facilitated by the action of the lubricating pressing aid. Owing to the deformation of the powder particles and the increase in packing density caused thereby, furthermore a portion of the pressing aid which is then in flowable state is displaced into the edge region, so that even when ejecting the finished green compact a considerably reduction in friction between the green compact and the wall of the compacting die is yielded. The softening temperature of the pressing aid must therefore be set such that, taking into account the operating temperature during the pressing operation, the outer surfaces of the green compact are not "moistened" by
the pressing aid, in order to avoid adhesion of loose powder particles.
Even at low molecular weight, there are no disadvantages upon mixing with the metal powder. The mixing operation during mixing into the metal powder and the softening point can be influenced to a certain extent by selecting the pressing aid and/or a mixture of pressing aids with corresponding molecular weight. Surprisingly, it has been found that polyethylene oxides even at very low molecular weights firstly can be mixed evenly with metal powders even in relatively small proportions by weight and secondly a good "flow" of the powder mixture is achieved upon filling the pressing mould and upon compaction.
The pressing aid can be mixed into the metal powder in the "cold" state, i.e. at room temperature. Hot mixing of the pressing aid with the metal powder is particularly expedient, for example in a heated drum mixer with subsequent cooling and simultaneous agitation, in which case the temperature of the mixer is initially set somewhat higher than the softening temperature provided for the pressing operation. The mixing temperature is expediently 50 - 100£C, preferably SS^C. After cooling, a flowable powder mixture is then available, which ensures good handling ability when filling the mould.
If the consistency of the pressing aid is liquid, it is possible to reduce the viscosity of the pressing aid further by means of an additional solvent, so that the powder particles can be coated even more thinly with the pressing aid in a process comparable to spray drying. Suitable solvents are in particular alcohols, such as ethanol, isopropanol or benzyl alcohol, which evaporate rapidly after spraying, so that the powder
obtained, to which the pressing aid has been added, is "dry" and the required trickling ability or flowability during pouring into the pressing mould is maintained.
In an advantageous embodiment of the invention, provision is made for the pressing aid to be contained in the mixture in a quantity of up to 5% by weight, relative to the content of metal powder. In this case, the fact that the density of the pressing aid according to the invention is higher than the density of conventional pressing aids is advantageously exploited, and thus, for the same proportion by weight, the pressing aid takes up less space and accordingly the compacted metal powder takes up more space. A content of pressing aid of at most 1% by weight relative to the metal powder is expedient.
The pressing aid in the form of polyalkylene glycols, in particular in the form of polyethylene glycols, is selected such that it has a softening point between 40^ and 80j7C. In this case, the use of polyethylene glycols having molecular weights of between 100 g/mol and 6500 g/mol, preferably 3000 to 6000 g/mol, has proved advantageous. In this case, mixtures of polyethylene glycols of different molecular weights are also expedient, but these then correspond approximately to the above total molecular weight in the mixture.
The hydroxyl value of the pressing aid may be between 500 and 700, while the density may be between 0.9 and 1.25 g/cm3.
Owing to the mixture of polyethylene glycols of different molecular weights, a pressing aid can be specifically prepared which can be matched accurately to the compaction process used in terms of mixture characteristics, softening point and lubricating
properties.
The pressing aid proposed here can be characterised by the following empirical formula:
(Formula Removed)
The increases in pressed densities which are to be achieved with the pressing aid stated here are not achieved primarily by means of a temperature-dependent change in the physical properties of the metallic powder, as in the method described in EP-A-0 375 627, but essentially by means of an improvement in the lubrication behaviour in the powder to be compacted itself, but in particular between the die wall and the powder filling given corresponding temperature.control at the pressing tools. A further advantage of the pressing aid proposed here consists in that it is thermally simpler to eliminate before sintering, for example by means of diffusion processes, escape by means of capillary action, sublimation, evaporation or the like. In this case, the pressing aid according to the invention is also distinguished by the possibility of environmentally friendly disposal, since it can be decomposed into water vapour and carbon dioxide by pyrolysis.
Surprisingly, it has been discovered that a mixture of a conventional amide wax, which is present as a very hard, brittle powder, also with a polyethylene glycol having a molecular weight of more than 7000 g/mol as pressing aid yields excellent compaction results and good ejection ability of the green compact from the pressing mould. "Wetting" on the outer surfaces is reliably avoided in this case. The proportion of polyethylene glycol in the pressing aid mixture may in this case be clearly below 40%. Ethylene pisstearoyl-
amide can be used as amide wax here.
Hitherto, metal stearates, in particular lithium or zinc stearates, and also paraffins, waxes, natural or synthetic fat derivatives have been used as lubricants to reduce the friction between the die wall and the powder particles on one hand and between the powder particles on the other hand. In more recent developments, multi-component high-temperature-resistant (i.e. in this case approximately 130gC) lubricants are used, which thus effect a reduction of the yield point of the metal to be compressed and consequently result in higher pressed densities, as also described in EP-A-0 375 627. A comparison of compressibility according to the various processes, conventional pressing at room temperature, so-called hot-pressing, as described in EP-A-0 375 627, with the method according to the invention is shown in the graph below.
For a test, a water-sprayed iron powder containing 2% copper and 0.6% carbon each in powder form was used. The curves indicate diagrammatically the dependency of the density on the compaction pressure.
Curve 1 as reference curve shows the result when using the cold-pressing method with a conventional lubricant in the form of an amide wax or microwax, for example ethylene pisstearoylamide.
Curve 2 shows the result when.using the hot-pressing method according to the cited prior art. Here, a considerable improvement can already be seen. In this case, however, the disadvantages described must be taken into account.
Finally, Curve 3 shows the result when using the method
according to the invention, which results in an even more considerable increase in final density.
In the following tables, the green densities and green strengths achievable depending on the compaction pressure are shown. In this case, the results which are obtained for different mixing processes for the metal powder to which pressing aid has been added and for different compaction pressures are compared.
Table 1
(Table Removed)

Table 2
(Table Removed)

Table 3
(Table Removed)

Table 4
(Table Removed)

Table 5
(Table Removed)

Table 1 here shows, for the above metal powder with a content of 0.6% by weight of polyethylene glycol having a molecular weight in the region of about 6000 g/mol which was mixed, and also compressed, in the cold state, i.e. at room temperature. The table shows an increase in green density and green strength which is practically proportional to the compaction pressure.
Table 2 shows the result for a starting material of the same composition, except that this was hot-mixed but cold-pressed. Here, in addition to an increase in the green density, there is a considerable increase in green strength compared with the values for cold compression of a cold-mixed powder. Mixing at a temperature in the region of the upper limit of the softening temperature of the pressing aid, or even somewhat above it, obviously yields a better distribution in the powder matrix and hence a thinner "lubricant film", which promotes the sliding movements of the powder particles and hence the "contact density" of the metal particles and the "interlocking" thereof which is made possible thereby.
Table 3 shows the values for a cold-mixed powder which is hot-pressed. The values which can be achieved for the green density correspond to the values given above, whilst the green strength shows a considerable increase, which shows the interaction between the type of polyethylene glycol of low molecular weight which is used and the temperature control during pressing.
In Table 4, for a hot-mixed metal powder which has been hot-pressed, there can then be noted a further increase in green density, with virtually the theoretically maximum density, close to the density of a solid iron, being achieved at a compaction pressure of 800 MPa. However, what is particularly striking here is the further increase in the green strength. The green strength was determined by a so-called 3-point bending test. The values given each designate the maximum specific applied load at which fracture of the green compact then occurs.
The improvement in green density, but in particular also in the green strength, which can be seen from the above tables is probably attributable to the use of a polyethylene glycol having a molecular weight of less than 7000 g/mol. What is decisive here is the increase in green strength which is noted upon hot-mixing, which is probably attributable to the fact that in the hot-mixing operation the iron powder particles, the copper particles and the carbon particles are coated with a very thin layer of the pressing aid. This can be seen from the fact that in a hot-mixed powder of the given composition the carbon powder which is to be mixed in does not dust and in a "finger test" does not stick to the finger, compared with the cold-mixed powder. Examination of the distribution of the powdered alloying contributions copper and carbon yielded a homogeneity which corresponds to the homogeneity of a diffusion-alloyed metal powder, as a metal powder in which first of all the iron powder and the powdered alloying constituents are mixed and the mixture is subjected to preliminary heat treatment in order to attach the alloy powder to the iron powder, so that segregation is avoided. Only after this does the admixing of the pressing aid occur in an additional operating step.
As the tests show, in the method according to the invention the energy-consuming preliminary heat treatment of the powder mixture can be dispensed with, since it is possible, in particular in the case of the hot-mixing process, to bind the powdered alloy constituents to the iron particles in non-segregating manner with good homogeneity by means of the pressing aid. This too demonstrates the advantage of the invention.
The increase in the green strength can probably be attributed to the better flow behaviour of the pressing aid with the relatively low molecular weight in the metal powder matrix under pressure and temperature, since, firstly owing to the very homogeneous mixture of pressing aid and metal powder and secondly owing to the thin "lubricant film" already obtained upon mixing, which is reduced still further upon hot-pressing, a much greater frequency of direct contact between metallic surfaces is provided between the individual metal particles and thus the plastic deformation and interlocking of the metal powder particles as described first hereinbefore can be obtained.
Somewhat higher values than those of Table 2 were surprisingly also yielded for a pressing aid mixture consisting of the amide wax and having a content of about 40% of a polyethylene glycol having a molecular weight of more than 6000 g/mol, which was mixed hot into the metal powder, which was then hot-pressed.
In Table 5, the values for the metal powder to which an amide wax was cold-admixed as pressing aid and which was cold-pressed are reproduced as a reference.

WE CLAIM:
1. A process for producing sinterable, metallic shaped parts from a
metal powder mixed with an auxiliary compacting agent, which
contains at least in part components from the family of
polyethylene glycols having a molecular weight of between 100 and
6,500 g/mol and which is filled into a compacting mold and, after
being compacted under pressure, is ejected as a compacted
shaped part from the mold.
2. A process as claimed in claim 1, wherein the pressing aid contains
at least one polyethylene oxide, in particular at least one
polyethylene oxide, in particular at least one polyethylene glycol.
3. A process as claimed in claim 1 or 2, wherein the pressing aid is
contained in the mixture in a quantity of up to 5% by weight,
preferably less than 1% by weight, relative to the content of metal
powder.
4. A process as claimed in one of claims 1 to 3, wherein the pressing
aid has a molecular weight of less than 7000 g/mol.
5. A process as claimed in one of claims 1 to 4, wherein the pressing
aid has a molecular weight of between 100 and 6500 g/mol,
preferably 3000 to 6000 g/mol.
6. A process as claimed in one of claims 1 to 5, wherein for a content
of polyethylene oxide of less than 40% in the pressing aid the
polyethylene oxide has a molecular weight of more than 6500
g/mol.
7. A process as claimed in one of claims 1 to 6, wherein the metal
powder to which the pressing aid has been added is poured into
the pressing mould at a temperature below the softening point of

the pressing aid used, so that softening of the pressing aid is produced, preferably during compaction.
8. A process as claimed in one of claims 1 to 6, wherein the metal
powder to which the pressing aid has been added is poured into
the pressing mould at a temperature below the softening point of
the pressing aid used and is compacted without energy being
supplied during pressing (cold pressing).
9. A process as claimed in one of claims 1 to 8, wherein the pressing
aid is mixed into the metal powder at a temperature which is at
least in the region of the softening point of the pressing aid (hot
mixing) .
10. A process for the manufacture of sinterable metal moulded parts
from a metal powder substantially as herein described with
reference to the accompanying drawings.